Exploring reaction mechanism for ammonia/methane combustion via reactive molecular dynamics simulations

Ammonia has gained growing attention as a carbon-free fuel. However, extensive studies are still needed to clarify and complete reaction mechanisms for ammonia combustion over a wide range of conditions. In the present study, a series of reactive molecular dynamics simulations that could reproduce the ammonia/methane (NH3/CH4) combustion in air at high temperature and pressure conditions were conducted. Results show that high temperature accelerates the consumption of NH3 and affects the yield of nitrogen oxides (NOx). Meanwhile, high pressure accelerates the NH3 consumption, which could be partly attributed to the frequent energetic collisions, and influences the formation of NOx. To understand the mechanism for NOx formation, the reaction pathways of NH3/CH4 combustion at high pressures were generated by tracing the trajectories of reacting atoms. Results suggest that high pressure complicates reaction pathways of NH3/CH4 combustion as it enhances the molecular/atomic collisions. By comparing with five popular NH3/CH4 combustion mechanisms, new intermediates and elementary reactions revealed by this study are identified. The activation energies for a few featured elementary reactions are calculated as well. The activation energies obtained from the present study agree with previous results from combustion kinetics studies. The present study demonstrates the feasibility to generate reaction network via tracing the atomic events in a reacting system and that the reactive molecular dynamics is a cost-effective tool in revealing in-depth reaction mechanisms under extreme conditions.

Automated summary

Ammonia has gained attention as a carbon-free fuel, but further research is needed to understand its combustion mechanisms under different conditions. In this study, reactive molecular dynamics simulations were conducted to study ammonia/methane combustion at high temperature and pressure. The results showed that high temperature accelerated ammonia consumption and affected the formation of nitrogen oxides (NOx), while high pressure also accelerated ammonia consumption and influenced NOx formation. By tracing the trajectories of reacting atoms, the reaction pathways for ammonia/methane combustion under high pressure were generated. Comparing with existing combustion mechanisms, new intermediates and elementary reactions were identified. The study also calculated the activation energies for several important elementary reactions, which were consistent with previous combustion kinetics studies. The study demonstrated the feasibility of generating reaction networks and revealed in-depth reaction mechanisms under extreme conditions using reactive molecular dynamics.